Zinc alloy plating has a better corrosion resistance than zinc plating, and hence has been widely used for automobile components and the like. Among types of zinc alloy plating, especially alkaline zinc-nickel alloy plating has been used for fuel system components required to have high corrosion resistance and engine components placed under high-temperature environments. An alkaline zinc-nickel alloy plating bath is a plating bath in which nickel is dissolved with an amine-based chelating agent selected to be suitable in terms of Ni co-deposition ratio, and zinc and nickel are co-deposited in a plated coating. However, when alkaline zinc-nickel alloy plating is performed, there arises a problem of oxidative decomposition of the amine-based chelating agent in the vicinity of the anode during current application. The oxidative decomposition of the amine-based chelating agent is caused by active oxygen generated at the anode. When ions of an iron group metal such as nickel ions or iron ions are coexistent, these ions act as an oxidation catalyst, and further promote the oxidative decomposition of the amine-based chelating agent. Accordingly, when an alkaline zinc-nickel alloy plating liquid comes into contact with an anode, the amine-based chelating agent rapidly decomposes, resulting in deterioration in plating performance. Accumulation of products of the decomposition causes many problems such as decrease in current efficiency, increase in bath voltage, decrease in plating thickness, decrease in nickel content in plated coating, narrowing of a permissible current density range for the plating, decrease in gloss, and increase in COD. For this reason, the plating liquid cannot be used for a long period, and has to be exchanged.
As methods for improvement in this point, some methods have been known so far. For example, Published Japanese Translation of PCT International Application No. 2002-521572 discloses a method in which a catholyte and an acidic anolyte in an alkaline zinc-nickel bath are separated from each other by a cation exchange membrane made of a perfluorinated polymer. However, when an acidic liquid is used as the anolyte, it is necessary to use an expensive corrosion-resistant member, such as platinum-plated titanium, as the anode. In addition, when the separation membrane is broken, there is a possibility of an accident in which the acidic solution on the anode side and the alkaline solution on the cathode side are mixed with each other to cause a rapid chemical reaction. Meanwhile, a plating test conducted by the present inventors has revealed that when an alkaline liquid is used as the anolyte instead of the acidic liquid, the anolyte rapidly moves to the catholyte upon current application, causing the lowering of the liquid level of the anolyte and the elevation of the liquid level of the catholyte simultaneously.
As a method for solving the above-described problems, Japanese Patent Application Publication No. 2007-2274 describes a method in which a cation exchange membrane is used, and an alkali component is supplemented to an alkaline anolyte. However, this method requires an additional apparatus, liquid management, and the like, which complicate the operations.
In addition, Published Japanese Translation of PCT International Application No. 2008-539329 discloses a zinc alloy plating bath in which a cathode and an anode are separated from each other by a filtration membrane. However, a test conducted by the present inventors has shown that the disclosed filtration membrane is incapable of preventing movement between the catholyte and the anolyte, and incapable of preventing decomposition of a chelating agent at the anode. In addition, since a zinc alloy plating liquid is used also as the anolyte, the decomposition of the anolyte is promoted very much. Accordingly, the anolyte has to be exchanged, and when the anolyte is not exchanged, the decomposition product moves into the plating liquid at the cathode. For this reason, it has been found that this method does not lead to substantial extension of the lifetime of the liquid.